232 THE LESSON OF EVOLUTION 



We relieve the fatigue, which comes on after 

 thinking intensely, not by ceasing to think, but by 

 changing the subject of thought. This shews that 

 it is the brain which cannot continue working, not 

 the mind. The energy in a certain portion of the 

 brain gets exhausted, and it is necessary to wait for 

 a new supply. Again, the object of sleep is to rest 

 the brain, not the mind; for the latter goes on 

 thinking when we are asleep. These thoughts, or 

 psychoses, when we are asleep, produce but little 

 physical changes, or neuroses, in the brain; for, 

 when we try to recollect our dreams, they generally 

 escape us. Often we know that we have been dream- 

 ing, but cannot remember even the subject. And 

 usually dreams, however vivid, rapidly become indis- 

 tinct, and we forget them altogether, unless they are 

 written down at the time. The more we work the 

 brain, the more liable we become to insomnia. We 

 have to try to stop thinking in order that the brain 

 may be rested, but the mind is inexhaustible. 



5. Mind, even in its lowest forms, is purposive, 

 or intelligent. That is, it has, or seems to have, 

 looked forward to results. All adaptations, such as 

 the formation of healing-tissues to cover over 

 wounds, must have a purposive origin. The fact 

 that these adaptations are helped by natural selec- 

 tion does not prevent them being purposive ; for the 

 purposive action is in originating the variation, not 

 in preserving it. Take the case of the origin of 

 chlorophyll. Everything, at that time, depended on 

 the reduction of the carbon-dioxide in the air. 

 Without that process life must have come to an end ; 



